1. Field of the Invention
This invention relates to coating compositions and, more particularly but not exclusively, concerns erasable coatings for xerography paper.
2. Description of the Prior Art
Erasure of a part of a xerography image from a paper carrier can be achieved by arranging for the adherence of the image to the carrier to be sufficiently weak that the part of it can be erased from the carrier. Alternatively, such erasure can be achieved if the carrier bears a coating to which the image adheres, and the coating can itself be selectively erased from the carrier substrate below. The present invention provides erasure of this latter type.
Essential requirements of an ideal such erasable coating include:
(i) Ease of erasure i.e. a firm adherence of the coating to the substrate, but which is readily broken by erasing action; PA1 (ii) Firm adherence of the original image to the coating; PA1 (iii) Drafting Receptivity i.e. ready permanent acceptance of ink and pencil impressions on and around the image area which has suffered erasure; and PA1 (iv) Image Breakthrough Resistance i.e. the coating must be unbroken and prevent any of the original image reaching the substrate, for any such penetrating image would not be erased by removal of the overlying coating.
It has already been proposed to provide xerography papers carrying an erasable coating. One example is the 100% rag-based vellum sold by Solvent Coating Corporation under the trade mark PERMALITE. The previously proposed coatings are based on a rubbery polymeric binder material in a solvent or aqueous carrier and may contain dyes or pigments and additives to modify the look and feel of the product paper.
The rubbery polymeric compositions are relatively expensive. It is one object of the present invention to achieve satisfactory standards in the essential requirements listed above, more cheaply than the previously proposed compositions.